The present invention relates to a golf ball excellent in flight performance and flight uniformity.
A multiplicity of dimples are generally arranged on the surface of a golf ball. Since the arrangement of these dimples exerts a large effect on flight performance of the golf ball, various attempts have been made to improve the shapes of the dimples and the arrangement of the dimples. For example, there has been adopted a dimple arrangement method including the steps of taking the spherical surface of the ball as a polyhedron such as an icosahedron, a dodecahedron, or an octahedron, uniformly dividing the polyhedron into polygons such as triangles, and uniformly arranging a plurality of kinds of dimples on each of the polygons as a unit, thereby uniformly arranging the dimples over the spherical surface of the ball.
Such a dimple arrangement method is effective from the viewpoint of uniformly arranging dimples on the spherical surface of a golf ball; however, it has a problem that the degree of freedom in dimple arrangement is not sufficiently large. To be more specific, since the dimples must be uniformly arranged in each of polygonal units divided from the spherical surface of the ball, there is a limitation in arrangement of the dimples in each of the polygonal units.
On the other hand, in producing a golf ball by injection molding, to form dimples on the surface of the ball, a large number of projections for forming the dimples must be provided on the inner wall of a cavity of a mold composed of upper and lower mold halves. In this case, since the parting plane of upper and lower mold halves is generally set to correspond to the equatorial plane of the ball formed by the cavity of the mold, it is difficult to form the dimples directly on the equator of the ball, and therefore, the projections for forming the dimples are generally not provided on the mold parting plane.
In the case of providing projections for forming dimples on a mold parting plane, the shapes of the dimples may be degraded when part of a golf ball molding material cured in a number of injection gates provided at positions of the mold parting plane is removed by polishing or the like. The provision of projections for forming dimples at positions on a mold parting plane may cause another problem in raising the cost required for design and production of the mold, and complicating the work of producing the mold.
Accordingly, projections for forming dimples are generally provided at positions out of a mold parting plane, with a result that an endless stripe-like land portion with no dimple is formed along the equator line of a golf ball, and thereby the continuation of the dimple arrangement is cutoff by the equator line of the golf ball, thereby making it difficult to continuously, densely form the dimples over the surface of the golf ball.
In the case of using a golf ball having such a dimple arrangement, the rotations of the golf ball differs depending on a position of the ball hit with a golf club. For example, the flight distance of the golf ball may greatly differ between the case of rotating the ball about a center axis connecting both the poles of the ball to each other and the case of rotating the ball about an axis extending in the direction perpendicular to the center axis. The golf ball having the above-described dimple arrangement may cause, in addition to a problem associated with a variation in flight distance, another problem that the ball is deviated rightwardly or leftwardly depending on the hitting position of the golf ball, failing to sufficiently obtain the direction stability of the ball.